The use of polycarbonate for twin wall sheets gives an outstanding product for use in the greenhouse industry. However, to provide an even better product, coatings can be applied to the polycarbonate or embedded in the polycarbonate.
For example, non ultra violet stablized (UV-stablized) polycarbonate is susceptable to UV-radiation, particularly in the range of 290-300 Nm. This problem can be overcome with the help of UV-stabilizers, either embedded in the polycarbonate or in a UV-stable toplayer (coated or co-extruded) thus damaging radiation is converted into heat.
These stabilizers can be added directly to the base material to avoid additional processing. However, this adversely effects the processability of the sheet and is very expensive. All that is effectively needed is a thin surface layer with a high concentration of UV-stabilizers.
This thin surface layer can be achieved either by coating or co-extrusion of the twin wall sheet. The coating of the twin wall sheets can be applied on line by spraying or by roller coating. Spraying works well with odd shaped objects yet the overspray results in loss of coating.
Roller coating has been used for coating flat sheets which need a one-side coating layer. For twin wall sheets, reversed rollercoating has been found to be both convenient and economical. The rollercoat is installed on line, followed directly by an oven. The oven is used to provide drying time. The length of the oven is dependent on the given speed of the extrusion line.
Surfactants have not typically been applied during this manufacturing process.
Surfactants have previously been sprayed on the sheets after the sheet has been constructed and cured. When spraying the surfactant on to the sheet, it is difficult to control a uniform thickness across the entire surface of the sheet. Also, the spray mist is not forced into the sheets. It has been found that this sprayed on surfactant can be washed off after a number of applications of water. These applications come from cleaning or condensation running off the sheets.
In polyethylene films, a substitute product for the structural sheets, the surfactant has been formed embodied in the product. Polyethylene films are typically made in large silos and are "blown" into a large roll. The surfactant is introduced into the polyethylene sheeting when it is in a molten state. However, it has been found that such surfactants can lose their moisture dispersion characteristics after approximately two years. However, with polyethylene films, this is not a problem since the polyethylene films need to be changed on a greenhouse environment after a period of two years.
It is, therefore, an objective of the present invention to provide an improved structural sheet having a long lasting surfactant applied to a portion of the sheet.
It is a further object of the present invention to provide an improved anti-fogging, moisture dispersing structural sheet which has a surfactant coated and embedded into the sheet.
It is still a further object of the present invention to provide a process to both embed and coat a surfactant on a plastic structural sheet so that the coat is uniformly applied to the sheet and is not easily removed by washing.
It is yet a further object of the present invention to provide a textured plastic structural sheet having a greater surface area over which to dispense a surfactant.
It is yet still another object of the present invention to provide a sheet with one surface co-extruded with an ultra-violet restrictive coating and a second surface with a surfactant embedded and coated on the surface, so that no lacquer coating is needed on the structural sheeting to restrict UV.
A process for treating the surface of plastic structural sheets with a long lasting surfactant to increase the longevity of the anti-fogging and moisture dispersion characteristics of the sheet is disclosed in the present invention. A product formed by this process is an anti-fogging, moisture dispersing structural sheet. The process encompasses exposing the surface of the sheet to a saturated solution of long lasting surfactant while the surface of the sheet is still tacky. The surfactant is pressed into the sheet with a series of rollers which also serve to meter the precise amount of coating of surfactant being applied to the sheet. In preferred embodiments of the present invention, structural sheets are pin-striped or fingered as the sheet leaves the die head. This fingering or pin-striping increases the surface area for the surfactant to be applied and also helps to channel moisture off the sheet.
The surfactant is applied after the sheet has been constructed yet prior to the time when the sheet has completely cured. At this state, the surfactant can be embedded within a portion of the outer surface of the sheet.
The surfactant is especially adapted for application to polycarbonate structural sheets used in greenhouse structures. In this environment, the surfactant helps to disperse moisture before damaging droplets form on the interior walls of the greenhouse. Otherwise, droplets can fall on plants within the greenhouse and cause disease, destroy foilage, and restrict transmission of light.
Other objects, advantages and novel features of the present invention will now become readily apparent upon consideration of the following descriptions of preferred embodiments in conjunction with the drawings.